In many high-frequency applications, a transmission line medium, as for example a coax cable or a strip line, is used so that one part of the system can be physically located at some distance from another part. Such an application could be, for example, the testing of a target device by a test system by connecting probes to the target device with transmission lines interconnecting the probes to the test system. Ideally such probes should detect and replicate the high speed electronic signals present in the target device with a minimum disturbance of the signal and a maximum fidelity of replication. These devices are commonly used for analyzing signals detected by electronic measurement equipment, including, for example, oscilloscopes and logic analyzers.
The usefulness of a probe depends upon the range of frequencies for which the response is true to the detected signal, the accuracy of replication, and the extent to which the probe detects the signal without detrimentally affecting the operation of the system or circuit being probed. If the input impedance of the combined probe and end-use device is the same order of magnitude as that of the circuit or system being probed, it may cause errors in the replication of the signal or a change in the operation of the circuit or system resulting in erroneous output or circuit malfunction. High probe tip capacitance will also cause circuit loading problems at higher frequencies. Designing the probe to have low capacitance and an input impedance which is high relative to the impedance of the circuit being probed at the point of probing has been the common protection against these errors. This high impedance results in only a small current to flow through the probe, allowing the target device to operate relatively undisturbed.
One measure of a probe is its intrusiveness or loading which is dependent upon the capacitance of the probe in parallel with the source resistance of the circuit under test. The capacitive reactance varies as a function of frequency resulting in the impedance of the probe also varying with frequency. The loading of previously available probes has been limited, because the impedance of the probes falls at high frequencies. Minimizing the capacitance of the probe has been one solution for reducing the loading of the probe. Compensating for probe tip capacitance by using active electronics at the probe tip is an alternative which has been used for extending the effective bandwidth of the probe tip. Such compensation, however, has generally resulted in a bulky and easily damaged probe tip.
Typical previously available probes included high resistance probes which minimized resistive loading and had high input impedance at D.C., but which had an impedance that fell off rapidly with increasing frequency due to high input capacitance. High impedance cable was used with these probes to minimize capacitance, but this cable was very lossy at high frequencies, limiting bandwidth. Such probes also required the measuring instrument to have a high impedance.
Also previously available were passive or resistive divider probes which had the lowest input capacitances available in a probe and therefore had a very broad bandwidth. However, the low input resistance could cause problems with resistive loading which could force the circuit under test into saturation, nonlinear operation, or to stop operating completely. Resistive divider probes in general do not have any inherent ability to compensated for transmission line losses.
Still other probes were active field effect transistor probes which had active electronics at the probe tip to compensate for loading problems due to low input impedance. These probes had a higher input impedance than the resistive divider probes and a lower capacitance than the high impedance probes, but were limited in bandwidth due to the field effect transistors. They were also bulky and easily damaged.
In other fields, a concept called pole-zero cancellation has been known. One application in which the concept was used was in a system for measuring heart rate disclosed by Lanny L. Lewyn in U.S. Pat. No. 4,260,951 entitled “Measurement System Having Pole Zero Cancellation”. In that system, pole-zero cancellation was used to cancel the long differentiation time constant so as to remove undesired shaping of the heart pressure wave caused by the second order feedback loop. This allowed the waveform to be refined so that it could enable greater accuracy in measuring the heart rate.
More recently, wide bandwidth probes with pole-zero cancellation have been utilized in probe tips. In U.S. Pat. No. 4,743,839 entitled“Wide Bandwidth probe Using Pole-Zero Cancellation” by Kenneth Rush, a pair of tip components and a pair of feedback components are utilized to obtain pole-zero cancellation. Values for the components are chosen such that a zero created by an RC circuit at the tip of the probe occurs at the same frequency as the pole created by a feedback circuit. The result of this design is probe circuitry that has constant gain over all frequencies.
In such high-frequency probing applications, a transmission line medium, as for example a coax cable or strip-line, is used so that the test equipment can be physically located at some distance from the target device to be probed. However, the transmission line impedance is typically low compared to that of the target device which can result in unacceptable loading of the target. To isolate the target device from the loading effects of the transmission line, passive networks at or very near the target are typically used. These isolation networks raise the impedance of the transmission line as seen by the target at the cost of reducing the signal strength as seen by the test equipment at the other end of the transmission line.
The transmission line medium typically has a measurable, frequency dependent insertion loss due to the effects of skin effect and a lossy dielectric medium. If insertion losses occur at frequencies low enough to be in the frequency band of measurement, these losses can result in an attenuation of the signal, as well as a distortion of the waveform with associated measurement error. It is therefore, desirable in many applications to compensate for these transmission line medium losses in order to achieve a higher usable bandwidth for the system.
The frequency characteristics of transmission line insertion losses are not simple poles. A generally accepted formula for describing the frequency characteristics of the insertion loss is a magnitude roll-off as a function of the square root of the frequency of interest. While pole-zero compensation circuits can be used to improve lossy transmission line response, the loss characteristic of the line is not a simple pole circuit falling off at 20 dB/decade. As such, the loss characteristics of the line cannot be adequately compensated for by simply using a circuit having a zero at the appropriate frequency.